The invention is thus based on high performance electric miniature motors. These are e.g. used in mobile telephones where a mass, which is mounted eccentrically on the shaft of the motor, upon rotation causes the mobile telephone to vibrate, whereby the user of the telephone is wirelessly made aware that there is an incoming call on the telephone. This illustrates a situation where the motor according to the invention may be used to advantage, and the example should not be taken to restrict the invention.
To provide a sufficiently high performance and speed for the motor, it is an advantage to use a permanent magnet as a rotor. This, however, normally causes problems in connection with motors of this size, since eddy current losses are created in the stator because of the rotating magnetic field that is established when the magnet rotates. In large motors, this problem is normally overcome by laminating the magnetic part of the housing/stator to the motor. Laminated magnetic cores are known for avoiding eddy current losses in magnetic cores in motors and transformers, etc., said core being composed of a plurality of individual sheets which are electrically insulated from each other at least partly, so that the stack of sheets has an at least significantly reduced electrical conductivity transversely to the planes of the sheets, that is in the direction of stacking.
In the field of high frequency technique it is known to use sintered magnetic elements of ferrite material.
The above solutions to the problem of eddy current losses in the stator of miniature motors have certain drawbacks. For example, lamination of the stator in an axial direction is rather expensive, since the stator of miniature motors has a very small wall thickness, and it adds considerably to the costs when the magnetic core of the stator has to be assembled from a plurality of small ring-shaped sheet elements with a sufficiently high precision.
The use of sintered elements is likewise inexpedient because of the necessary small thickness of material, since the stator hereby gets a too low ultimate strength, and also it is difficult to make a stator of a sintered material, such as ferrite, with sufficiently narrow tolerances.
A miniature motor of the above-mentioned type is known from an article or advertisement Tiny motors packed with precision published in Machine Design, Jul. 9, 1998, page 58. This motor, however, also has drawbacks. The motor uses a wound coil made by a process wherein the coil threads must be wound very closely together and be positioned with great precision because of the small size in order to achieve a suitably high performance in the very small and compact coil. A very high degree of precision is required for such a process, which makes the process cumbersome, and the resulting motor is therefore expensive.
Furthermore, this known motor has no commutator brushes. This requires additionally complicated and cost-increasing control electronics which is used for keeping track of and determining the angular position of the rotor relative to the stator coil, so that the correct part of the coil can be activated to affect the magnet expediently. The high speeds of rotation moreover make very great demands on the control electronics. An additional drawback of the use of the external control electronics is that the electronics per se takes up much space relative to the miniature motor. This manifests itself particularly in connection with mobile telephones where size, weight and price are very much competitive parameters.
The object of the present invention is to provide a compact high performance miniature motor with a high speed of rotation, which also consists of simple components and is thereby easy and simple to manufacture.
The invention provides a high performance miniature motor with a low eddy current loss and containing quite few simple elements, such as bearings, a double-sided flexible circuit board provided with coil parts on both sides, a spring used as a commutator brush, a permanent magnet used as a rotor, etc. This allows a very simple and inexpensive manufacturing method for the motor.
The problem of eddy currents in the housing when the magnet rotates is solved in the following manner according to the invention.
In a preferred embodiment, the motor housing is formed by a plurality of non-plane subelements which are provided with protruding parts and corresponding cavities to receive protruding parts on another subelement so that the subelements have a self-centering effect when assembled. This means that the housing is essentially electrically insulating in the longitudinal direction of the motor, since, in practice, the division into subelements gives an electrical division into layers which, otherwise, can typically be achieved only by an expensive lamination, so that eddy current losses are avoided entirely or reduced considerably. The housing is still magnetically conductive, as is required for a stator in a motor.
According to a preferred embodiment, the electrical terminal wires are integrated on the flexible circuit board. This ensures an additionally simple and flexible structure, as the terminal wires do not have to be soldered on the coils, but have already been integrated. This gives an easy and inexpensive mounting of the motor in e.g. a mobile telephone.
In an expedient embodiment, the end part of the spring touching the commutator segments has a direction of winding opposite to the direction of rotation of the shaft. This ensures that the spring cannot inexpediently lose contact with the shaft, since the spring will be tightened additionally when the motor is used.